The ships were a substantial improvement over the preceding Trento-class cruisers, incorporating significantly heavier armor protection at the cost of the very high speed of the Trentos.

After Italy joined the wider conflict in 1940, the four ships saw extensive action in the Mediterranean Sea against British forces, including the battles of Calabria and Cape Matapan.

During the latter engagement, Pola was immobilized by an aerial torpedo, and then along with Zara and Fiume were all sunk in a one-sided night action with three British battleships.

While the preceding Trento class of heavy cruisers were still being built, elements of the Italian naval command began to doubt the effectiveness of the new vessels, which sacrificed armor protection in favor of very high speeds.

They advocated a more balanced design that would incorporate more comprehensive armor, with a main belt that was 200 millimeters (7.9 in) thick, while retaining the battery of eight 203 mm (8 in) guns and a speed of at least 32 knots (59 km/h; 37 mph).

The naval command agreed to allow the new ships to exceed the displacement limits, but instructed the designers to eliminate unnecessary features to save as much weight as possible.

The first three ships were built with light superstructures as a weight saving measure, but Pola, intended to serve as a flagship, received a much larger bridge structure to accommodate an admiral's staff.

The ships carried a pair of IMAM Ro.43 seaplanes for aerial reconnaissance; the hangar was located under the forecastle and a fixed catapult was mounted on the centerline at the bow.

The vessels each carried 2,300 to 2,400 long tons (2,300 to 2,400 t) of fuel oil, which allowed them to steam for 4,850 to 5,400 nautical miles (8,980 to 10,000 km; 5,580 to 6,210 mi) at a cruising speed of 16 knots (30 km/h; 18 mph).

The problem was in large part due to poor quality control in Italian munition factories, which failed to ensure tight manufacturing tolerances necessary for accurate shells.

[4] In addition, the single cradle mounts required the guns to be very close together; this caused the shells to interfere with each other in flight and contributed to the dispersion problem.

After departing France, Gorizia visited Germany, and on the way back, she suffered a major explosion in an aviation gasoline tank that forced her to put into Gibraltar for repairs.

In March 1939, all four ships sortied to prevent a squadron of Republican warships from reaching the Black Sea, forcing them to stop in Bizerte, Tunisia, where they were interned.

[12] The Zara-class cruisers saw extensive service during World War II, having taken part in several sorties to catch British convoys in the Mediterranean as the flagship of the 1st Division.

[12] At the Battle of Calabria in July 1940, torpedo bombers from the British aircraft carrier HMS Eagle attacked the Zara-class cruisers, but they failed to score any hits.

The ships steamed ahead to the front of the Italian line of battle and joined the attack on the leading British battleship, Warspite, but they scored no hits.

[12] In March 1941 at the Battle of Cape Matapan, Pola was immobilized by a torpedo from a Swordfish torpedo bomber launched by the British aircraft carrier HMS Formidable; Zara and Fiume were detached from the rest of the fleet to protect Pola, and all three and a pair of destroyers were sunk in a close-range night engagement with the battleships Barham, Valiant, and Warspite.